Rueter forecast 9

Nik Rueter

4-16-12

The forecast for the Boston and surrounding areas looks to be dry weather with above average warm temperatures for the workweek. A deep low will continue to track northeast into Canada and slowly drag the cold front through our area early tomorrow morning. Unlike the heavy precipitation and thunderstorms that was associated with this system over the Midwest and Ohio River Valley yesterday, by the time it gets to our area it will be a dry front. This is due to to the very low dewpoints in our area, and this dry air mass will prevent any positive forcing and moisture/instabilty from building in. Convection is therefore very unlikely as temperatures are already in the high 70s with dewpoints in the upper 40s. Clear skies and full sun will allow temperatures today to reach record highs in the interior, into the upper 80s, while the coastal temperatures will be a bit lower due to an east wind blowing onshore from the ocean.

Soundings for the area show dry midlevels and subsidence that will quickly fill in as this high pressure moves closer to area. Strong frontogenesis will be present with the passage of the cold front, but as mentioned before, the air mass will not be able to saturate enough to initiate convection. Temperatures will be a bit cooler as the cold front passes in the low 60s for Wednesday, with cold air advection taking effect by Wednesday along with the anticyclonic flow and northerly winds.

A weak shortwave will pass through on Wednesday evening, but will little to no moisture, conditions will be continued dry and sunny. Expect temperatures Wednesday through Friday to be closer to normal, in the mid 60s. Clouds will be on the increase for the weekend as a trough digs into the Southeast U.S. QG forcing looks to be good for this system, and southerly flow ahead of this cyclone will give our area a good chance of rain with the positive forcing associated with the PVA and WAA ahead of the trough. However, there is a lot of uncertainty with the track of this system for the weekend, so the speed and track of it could greatly impact the timing and amount of precipitation that we receive.

So the main focus for the short term will be the record warm temperatures, the passage of the cold front and cold air advection that follows, and the deep trough that will build in by the weekend giving our area a good chance of rain.

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Verification

4-19-12

Since the primary focus of the forecast was record high temperatures and the chances of preciptation, let's begin there. At the time of the forecast on the 16th, temperatures for the next two days were forecasted to be record highs with very low humidity. A cold front passed through the morning of the 17th, and no precipitation was forecasted due to the very low dew points. Precipitation also looked to be possible on Wednesday evening with the passage of a weak shortwave trough, but none was forecasted due to the continued low dewpoints and lack of moisture to overcome this. These were both accurate forecasts, as no rain fell from the forecast time through now for the Boston area.

As far as temperatures go, a record high of 87 degrees was set for Boston on the 16th, which is 31 degrees above their average! A distinction between highs on the coastal cities and inland was made, saying that temperatures would be warmer inland, because of the lack of land-sea breeze. I am very surprised at just how much of a difference there was. Coastal cities of Boston and Providence had highs of 87 and 83, respectively, while Hartford, which is inland, had a high of 92! This is a significant 5-10 degree difference in tempertures over a very short distance. Temperatures on the 17th were continued warm at 84 for Boston. Common sense would say that after the passage of a cold front temperatures would be much colder, but this was not the case and was forecasted well by the models as temperatures stayed in the mid-80s still. A significant cooldown was seen on the 18th, as the high for Boston was only 62. The same can be expected today as temperatures are currently in the low 60s with partly cloudy skies. Dew points are also still very low, in the mid 30s, which is indicative of how dry this stable air mass has been that prevented any precipitation earlier this week.

The models handled the chances of precipitation, the timing of the cold front passage and its effect on temperatures very well. High temperatures were forecasted spot on, and the land-sea breeze interaction was highly noticeable. The change to the dry forecast came on Friday into the weekend as a trough digging into the Southeast US looked to be providing positive forcing and moisture to this area for the weekend.

There is a significant chance in the models for the track of this low, as on Monday it looked like it would dive south and then track northeast along the coast, bringing significant rains to the Boston area. The 500mb vorticity plot of the 12Z run of the NAM today shows that this low will actually become completely cut off from the main flow and eject into the Gulf of Mexico. There is still a good chance of showers for the weekend associated with an upper level long wave trough, although it will be more in the Saturday night into Sunday timeframe. The track and speed of these cutoff lows have been a problem with the GFS this whole year, and this case was no different. While the short term forecast was very accurate, the five to six day mark had big changes as far as the track of the low pressures, and highlights the importance of doing frequent updates with different model runs.